1. Field of the Invention
The present invention relates to a pipe-bending machine mandrel.
2. Description of Related Art
The mandrels are devices that are used inside pipes to be bent in order to prevent defects and distortions of the pipes in a bending operation.
By way of example, U.S. Pat. No. 5,909,908, which was granted on Jun. 8, 1999, describes a combined punching and bending apparatus to punch and bend a pipe, in which a core bar that is supported at an end of a rod comprises a main body and two rigid members, which are articulated to the main body in sequence and are designed to tilt together with a correspondent section of the pipe being bent.
Such rigid members, which are designed for rectangular pipes in the cited document, are of prismatic shape, are chamfered at their ends, and have transversal dimensions that are slightly less than the dimensions of internal cross-section of the pipe to allow the mandrel to slide with respect to the pipe. In the bending operation, the internal walls of the pipe, i.e. both the proximal one and the distal one with respect to the bending centre, are in contact with the rigid articulated members only along a line, since the rigid articulated members have straight side surfaces, and the pipe is bent according to any desired bending radius Rm. Bending radius Rm means the distance as measured between the centre of a bending die, by which the pipe is bent, and the external edge of the same bending die, i. e. between the centre of the bending die and the centre of the pipe to be bent.
However, a large range, in which the same mandrel is used with different-radius bending operations, has, as a negative set-off, distortions and defects that are shown by the generation of flattening, bulges or wrinkles in the bent section of the pipe. In other words, the same profile that a bent pipe had before the bending operation cannot be kept after it.
Japanese Patent Application No. 2005-205482, that was filed on Jan. 26, 2004, describes a pipe-bending mandrel having a tilting member in its front end opposite to a mandrel holding rod, tilting member which is pivoted between two side ends. In their distal side, i. e. in the side far from the rotation centre of the bending die, the side ends are configured like a surface that the pipe will get on that side at the end of the bending operation.
In the above cited Japanese Patent Application the tilting member has a substantially straight distal side, which is parallel to the pipe before the pipe is bent. This distal side is radiused both in front and at the rear to abut against the pipe on two points. The proximal side of the tilting element is conformed like a surface which the pipe will exhibit on that side at the end of the bending operation. In such a way, the pipe to be bent is not supported suitably throughout the bending operation. Further, the mandrel consists of mutually moveable parts, and this makes the mandrel weak and wearable over the time. Furthermore, the mandrel according to the cited Japanese Patent Application has one working position whereby an operator has to be careful that in operation the tilting member is perfectly coplanar with the bending die.
To overcome the drawbacks of the prior art, an object of the present invention is to manufacture a pipe-bending machine mandrel having either an integral rigid member or a rigid member which is comprised of a plurality of parts which are securely connected together in one body, that depends on a radius of a curve at which a section of pipe has to be bent.